Wattage is a measure of work which may be alternately expressed in horsepower. Since Wattage is an electrical quantity, it can be electronically calculated and converted into units of horsepower. To calculate wattage drawn by an electrical device, a wattmeter may multiply the current flowing through the device by the voltage drop across the device. The multiplication must be calculated at every instant in time when alternating voltages and currents are present. Converting the wattage drawn by an electrical device to a value of horsepower is a simple mathematical multiplication accomplished by scaling or amplifying.
Horsepower and wattmeters are commonly used to determine the power dissipated in electrical loads, such as large electrical motors, lathes, machine tools and the like. Monitoring the power requirements of such devices is useful for speed control, recording operational parameters, maximizing operational efficiencies, preventive maintenance and the like. Historically, wattmeters have been galvanometer types which make use of the magnetic interaction between a moving electromagnet on the pointer of the meter and stationary electromagnets. The multiplication of current and voltage necessary to determine power is accomplished by mechanically passing these two coils through one another's electromagnetic fields. This type of meter has a number of serious drawbacks. First, perfect coupling cannot be achieved between the electromagnetic fields and, thus, inaccuracies are introduced into the meter. Secondly, this type of meter does not lend itself well to interfacing with electronic equipment, such as process controllers or recorders, due to the heavy current which is necessarily passed through the meter.
A second and more modern type of wattmeter uses the Hall effect principal in an integrated circuit surrounded by the same type of electromagnets as in the galvanometer wattmeter. This type of wattmeter can be interfaced with electronic equipment, but shares another drawback with the galvanometer. Any wattmeter must sense current flow into the device whose wattage is being measured, and in the case of the Hall effect meter or the galvanometer, this current must flow through heavy windings (electromagnets) within the instrument. When the current flow is too great for the meter's internal windings to carry, an external current transformer must be used which introduces greater cost, greater instrument error and restricts use to AC applications only. Additionally, wattmeters are dangerous to use, in that extremely high voltages and currents are flowing through exposed terminals on the instrument and handling of the instrument during operation, such as for changing meter scaling and the like, can be extremely hazardous. Still another shortcoming is the physical size of wattmeters. The size is necessitated by the large current coils contained within the instrument's housing, as well as the physical bulk of the meter movement itself. Finally, some wattmeters are not universally adaptable for AC and DC operation without adjustment and/or modification.